This application claims the priority of German application 197 49 295.9, filed in Germany on Nov. 7, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a direct-injected four-cycle internal combustion engine. Especially preferred embodiments relate to a direct-injected, four-cycle internal combustion engine with at least one cylinder, in which a combustion chamber is delimited by a piston, movable lengthwise in a respective cylinder, and a combustion chamber roof is formed by an interior of a cylinder head, with one injector per cylinder for injecting a conical fuel stream into the combustion chamber, with a fuel/air mixture being formed with combustion air supplied separately, said mixture being ignited by an igniting spark jumping between electrodes of a spark plug.
The internal mixture formation of the fuel sprayed into the injection chamber with combustion air supplied separately allows stratified charge operation, which achieves a significant reduction in fuel consumption by comparison with other known mixture formation methods over broad partial load ranges of the engine. The stratified combustion chamber charge is produced by injecting fuel during the compression stroke of the piston, with an inhomogeneous stratified mixture with a high fuel concentration in the vicinity of the fuel stream forming during the short interval for mixture preparation between the injection of the fuel and the ignition of the mixture. Hence, the combustion chamber contains areas of ignitable fuel/air mixture in the form of mixture clouds around which the mixture ratios become increasingly lean so that the four-cycle internal combustion engine can be operated with surplus air. A stable charge stratification that permits extensive de-throttling in the partial load range and even throttle-free operation during idle, is achieved by injecting a conical stream of fuel, with the injector being equipped with a suitable injection nozzle.
The mixture cloud is ignited by an igniting spark that jumps between the electrodes of a spark plug. The ignition of the stable stratified charge with surplus air often is achieved with a spark plug located immediately adjacent to the injector, ensuring that an ignitable, i.e. fuel-rich mixture will be contacted by the igniting spark.
A four-cycle internal combustion engine of this type is already known from German Patent Document DE 43 24 642 A1. The ignition and burning of the mixture takes place during stratified charge operation soon after fuel injection ends and before an increasing homogenization of the stratified mixture takes place, and the ignitable mixture cloud advances in the direction of the fuel stream away from the electrodes located near the injectors. The electrodes of the spark plug in the known arrangement extend deep into the combustion chamber and into the interior of the conical fuel stream that is produced during injection. The electrodes are wetted with fuel during injection which cannot fully evaporate before ignition occurs, causing deposits on the electrodes during the combustion process. The increasing coking of the electrodes causes surface discharges and thus misfiring, and the spark plug rapidly becomes nonfunctional. In addition, with a "stream-guided" direct-injection concept of this kind, in which the fuel is delivered to the electrodes near the injector by the movement of the flow in the fuel stream, only a short time interval is available for preparing the mixture before ignition, so that soot deposits caused by incomplete fuel combustion due to insufficient mixture preparation can frequently be seen.
German Patent Document DE 195 46 945 A1 teaches a direct-injected four-cycle internal combustion engine in which the electrodes of the spark plug are located outside the conical jacket of the fuel stream and therefore are not wetted with fuel during injection, so that coking is prevented. The roof of the combustion chamber is designed to be roughly conical and extends at a distance parallel to the conical jacket. Through the air gap formed between the fuel cone and the roof of the combustion chamber, an air stream which was previously displaced by the injection stream is intended to flow backwards against the flow direction for reasons of continuity and to trigger a vortical flow which entrains fuel droplets or a mixture from the conical jacket and transports it to the electrodes. The flow-mechanical design of the mixing vortex into which the electrodes must extend, however, cannot be controlled. With a limited vortex strength in the vicinity of the spark plug, it is not possible to remove enough fuel from the conical jacket to bring a mixture capable of igniting between the electrodes. Misfires are the unavoidable result.
A goal of the present invention is to design the internal combustion engine according to the species in such fashion that sufficient preparation as well as stable ignition and combustion of the fuel/air mixture are reliably guaranteed during stratified charge operation.
This goal is achieved according to certain preferred embodiments of the invention by a direct-injected four-cycle internal combustion engine of the above referred to type, wherei the electrodes are located outside the conical jacket of fuel streams, and wherein the combustion chamber roof is so designed that the conical jacket is formed during injection in a portion that extends between the injector and the spark plug immediately adjacent to a wall of the combustion chamber roof.
The design of the combustion chamber roof according to the invention makes it possible to supply a fuel-rich mixture despite the electrodes being located away from the conical jacket. The fact that the conical jacket is so close to the wall of the combustion chamber roof causes an outwardly directed deflection of the fuel droplets guided in the conical jacket. The spreading of the fuel stream is influenced by the laterally close combustion chamber wall, with a vacuum developing at the fuel chamber wall that attracts the flow layers of the conical jacket. This phenomenon of deflection of a flow formed adjacent to a solid is termed the Coanda effect, which permits the transport of ignitable mixtures to the spark plug by simple means. Due to the extent of the conical jacket length in the generating line direction and the width of the stream area that is guided close to the wall of the roof of the combustion chamber, the degree of deflection in the direction of the electrodes of the spark plug can be varied. By utilizing the Coanda effect, the distance between the injector and the electrodes can be increased so that before ignition takes place, sufficient mixture preparation takes place and soot formation is reliably avoided.
Advantageously, a guide rib rises between the injector and the spark plug, running approximately parallel to the generating line of the conical jacket, said rib causing the radial deflection of the conical jacket toward the spark plug and not reducing the combustion chamber volume at all.
The guide rib can extend as far as a receiving opening of the spark plug in the roof of the combustion chamber, reinforcing the deflection caused by the Coanda effect. If the electrodes are located in the mouth area of the receiving opening of the spark plug, they are reliably protected against being wetted by the fuel by the soffit of the guide rib.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.